Not Applicable.
Not Applicable.
1. Field of the Invention
The present invention relates to casing joint locator devices of the type used within wellbores. The invention also relates generally to devices and methods for detecting connections in strings of tubular members by sensing perturbations in natural magnetic fields induced within the string.
2. Description of the Related Art
Casing collar locators are used to locate joints within the borehole casing. The locator is suspended on a wireline cable and passed through the cased borehole. The locator device detects the collars used at joints in the casing string as the locator device is moved upwardly and/or downwardly through the casing. Sections of casing are typically joined by an exterior collar which secures the two adjacent ends of the casing section to one another in a threaded engagement. As the locator moves adjacent to a collar, it detects a change in the magnetic readings resulting from the increased exterior thickness of, or additional mass of metal associated with, the casing wall.
Casing collar locators are extremely important tools for downhole operations. They are virtually required for depth correction operations and for the accurate placement of downhole devices such as locks and packers. It is desired to avoid the location of a casing joint when setting a packer, for example, since the joint presents a gap or discontinuity in the casing wall that may prevent the packer""s elastomeric sealing element from sealing properly at those locations.
In order to detect a casing collar, conventional casing collar locator devices typically rely on the generation of a relatively powerful magnetic field from the locator using either a permanent magnet or by passing a current through a coil to induce magnetism. A significant amount of power is required to generate the magnetic field. As the coil passes adjacent a collar in the casing, the flux density of the magnetic field is changed by the additional thickness of metal provided by the collar. The change causes an electrical output signal to be generated that indicates the presence of the collar, and this output signal is transmitted to the surface of the well through the wireline.
Unfortunately, conventional casing collar locator devices suffer from operational disadvantages and limitations of their effectiveness. Conventional locators are not greatly sensitive, in general, to changes in the wellbore casing. As a result, conventional casing collar locators are reliable only in a xe2x80x9cdynamicxe2x80x9d mode wherein the locator is moved rapidly through the wellbore casing in order to accurately detect the presence of collars. If the locator is moved too slowly, the changes in the signal indicative of the presence of the collar may be too gradual to be recognized by the well operator.
Dynamic location of collars is disadvantageous because it tends to provide less accurate real-time information concerning the position of the casing joint. For example, if it is desired to set a packer five feet below a particular casing joint collar in a wellbore, a conventional casing collar locator would be moved rapidly either upwardly or downwardly through the wellbore until the particular casing collar is detected. When that occurs, a signal is provided to the wellbore operator which indicates the location of the collar. Due to movement of the locator through the casing, however, the casing collar locator is no longer positioned proximate the collar by the time the operator receives the signal and reacts to it by stopping movement of the locator. The precise position of the collar must then be somewhat approximated given the current position of the locator within the wellbore.
Additionally, conventional locator devices locate casing joints by detecting a difference in thickness of the casing wall due to the presence of an external collar. These devices are actually, xe2x80x9ccollarxe2x80x9d locators rather than xe2x80x9cjointxe2x80x9d locators. As a result, they are unable to reliably detect a xe2x80x9cflushxe2x80x9d joint wherein the casing wall thickness is not appreciably altered by the presence of the joint. A flush joint can occur where the adjacent casing sections are threaded directly to one another or where the collar is unusually thin or contains very little metal.
In addition, because conventional casing collar locators generate a significant magnetic field, they tend to interfere with other downhole instrumentation that rely upon accurate magnetic readings. For example, a compass-type magnetometer that is attempting to find magnetic north can be confused by the magnetic field generated by the casing collar locator. Some induction-type locators are known that generate and transmit strong electromagnetic waves, rather than magnetic fields, to detect casing collars. Unfortunately, these devices also tend to interfere with downhole instrumentation.
A need exists for a casing collar locator device that can more reliably detect the presence of casing section joints in a wellbore and flush joints that do not employ radially enlarged collars. Further, a need exists for a locator that generates a minimal or no magnetic field that would affect the operation of other downhole instrumentation.
The present invention provides an improved casing joint locator that is capable of reliably detecting joints between sections of casing in a wellbore. Methods and devices described herein sense perturbations, or changes, in magnetic fields that are induced in the casing sections by the earth""s natural magnetic field. The induced magnetic fields include attractive forces that result from magnetic fringe effects proximate the longitudinal ends of the casing sections. The attractive forces are present at the connective joints of the casing string, thus presenting perturbations in the magnetic fields associated with the casing. Therefore, the inventive methods and devices will detect voids, such as air gaps and discontinuities, associated with a casing joint as well as the external collar associated with a standard collared joint. Thus, the inventive methods and devices are capable of detecting flush joints as well as more traditional joint arrangements.
The devices and methods described herein are also capable of providing clear and reliable signals indicative of the presence of flush joints wherein there is no appreciable change in the diameter of the casing at the joint. As a result, the possibility of a well operator failing to recognize such a signal is minimized.
The casing joint locator of the present invention generates essentially no magnetic or electromagnetic field. As a result, other downhole instrumentation is not affected by its presence. The locator device relies upon the earth""s natural magnetic field to polarize and, thus, induce a magnetic field in the surrounding casing sections. Perturbations in this naturally-induced magnetic field, such as will result from the fringe effects associated with air gaps or discontinuities in the casing are detected by the locator device. The magnetic signature associated with the presence of a surrounding casing collar is also easily detected by the locator.
Further, methods and devices of the present invention provide for accurate measurement of lengths and distances, such as the length of casing joints or the distance between such joints.
The present invention also provides a locator device having a very small physical size and which uses very little power. Further, the locator device of the present invention does not need to be moved rapidly through the wellbore in order to reliably detect a casing joint. Thus, methods are described for xe2x80x9cstaticxe2x80x9d detection of casing joints wherein the locator is moved either very slowly or not at all, and the casing joint can still be reliably detected.
Thus, the present invention comprises a combination of features and advantages which enable it to overcome various problems of prior devices. The various characteristics described above, as sell as other features, will be readily apparent to those skilled in the art upon reading the following detailed description of the preferred embodiments of the invention, and by referring to the accompanying drawings.